Release material, release material article, and process for producing the release material article

ABSTRACT

The present invention provides a release material formed by irradiating a release material precursor having a shear storage modulus of about 1×10 2  to about 3×10 6  Pa at 20° C. and a frequency of 1 Hz, wherein the release material has a contact angle of 15° or more, measured using a mixed solution of methanol and water (volume ratio: 90/10) having a wet tension of 25.4 mN/m. The release material of the present invention has improved anchoring to certain substrates, ensuring relatively low release strength from certain pressure-sensitive adhesives, particularly even after the exposure to a high temperature. Thus, the release materials are capable of allowing the pressure-sensitive adhesive to maintain stable residual adhesion strength.

FIELD OF THE INVENTION

[0001] The present invention relates to a release material, a releasematerial article using the same, and a process for producing the releasematerial article.

BACKGROUND OF THE INVENTION

[0002] Pressure-sensitive adhesive tapes and pressure-sensitive adhesivesheets are usually obtained by providing a pressure-sensitive adhesiveon the surface of a substrate. In the case of tapes, the article istypically stored in a roll form. On the back surface of the substrate, arelease material is usually coated so as to protect the adhesive surfaceand facilitate unwinding upon use. In some cases, another substratehaving a release material coated thereon is used to cover and, thereby,protect the adhesive surface.

[0003] In general, release materials are roughly classified into thosematerials containing silicone (“silicone-type release material”) andthose materials not containing silicone (“non-silicone type releasematerial”). Silicone-type release material can provide a releasestrength over a wide range, from relatively low (e.g., 0.1 N/25 mm orless) to relatively high (e.g., 10 N/25 mm or more). However,silicone-type release material may readily transfer to an adhesivesurface due to insufficient anchoring to the substrate, for example, aplastic substrate. The transferred silicone-type release material maygive rise to contamination. For examples, in the electronics industry,this may be undesirable.

[0004] Non-silicone type release material is further classified intothose materials containing fluorine (“fluorine-type release material”)and those materials containing a long chain alkyl group (“long chainalkyl group-type release material”). Fluorine-type release materials aregenerally relatively expensive and the types of solvent that can be usedfor diluting and coating fluorine-type release materials are typicallylimited to fluorine-type solvents due to solubility issues. Therefore,practical use of fluorine-type release materials is limited. Long chainalkyl group-type release materials typically consist of a copolymer ofan acrylic ester, vinyl ether, and acrylamide derivative or the like andthese are disclosed, for example, in Japanese Examined PatentPublication (Kokoku) Nos. 44-9599, 40-8903 and 43-21855. Morespecifically, Japanese Examined Patent Publication (Kokoku) No. 44-9599discloses a release material consisting of a copolymer of octadecylmethacrylate and acrylonitrile. Japanese Examined Patent Publication(Kokoku) No. 40-8903 discloses a release material consisting of acopolymer of octadecyl vinyl ether and acrylonitrile. Also, JapanesePatent Publication (Kokoku) No. 43-21855 discloses a release materialconsisting of a copolymer of octadecyl acrylamide and acrylic acid.

[0005] However, these release materials are poor in anchoring to certainsubstrates, such as polyester. Furthermore, these release materialsusually contain a group having high affinity for a pressure-sensitiveadhesive, such as carboxyl group, nitrile group, amide group, orhydroxyl group. Such a group may drive the release material to readilytransfer to the interface with, or into, an adjacent pressure-sensitiveadhesive, or vice versa. As a result, the release material tends to bedifficult to release from the pressure-sensitive adhesive or theadhesive properties, such as adhesive strength, of thepressure-sensitive adhesive tend to be decreased. These tendencies aregenerally more pronounced when the release material is heated to highertemperatures.

[0006] Japanese Unexamined Patent Publication (Kokai) No. 63-202685discloses an acrylic release material obtained by crosslinking anacrylic copolymer of an acrylic ester and an acrylic compound having afunctional group with a crosslinking agent. According to thispublication, a crosslinking agent such as isocyanate is used for acrosslinking treatment. Crosslinking reactions involving isocyanatestend to proceed gradually and, thus, an undesirably high amount of timemay be necessary for the desired performance, such as releasingstrength, to be obtained in such materials. Furthermore, theabove-described acrylic copolymer contains a polar group such ascarboxyl group, hydroxyl group, or amide group, which may provide ahigher release strength as compared with release strengths ofsilicone-type release materials. In addition, when an acrylic copolymeris crosslinked using an isocyanate, it tends to be difficult to releasethe release material from an acrylic pressure-sensitive adhesive. Thistendency is generally more pronounced at a higher temperature.

[0007] Japanese Unexamined Patent Publication (Kokai) No. 11-152459discloses a non-silicone type release material consisting of an acryliccopolymer that does not contain a polar group. This release material isobtained by polymerizing a release material forming compositioncontaining (a) an organic compound having two or more (meth)acryloylgroups and having a number average molecular weight of 600 or more and(b) an alkyl (meth)acrylate under the irradiation of an active energyray, such as ultraviolet ray or electron beam, while forming crosslinksthrough the (meth)acryloyl group, and curing the polymer. The releasematerial of this publication contains no polar groups and even whenexposed to high temperatures, the release strength is presumed to notsignificantly increase. However, organic compounds having two or more(meth)acryloyl groups do not effectively decrease the release strength.Although the monomer unit of alkyl (meth)acrylate, which is contained inthe copolymer after curing, has a tendency for decreasing the releasestrength of the release material, the monomer of alkyl (meth)acrylatecontained in the release material forming composition is not completelypolymerized, even under irradiation of an active energy ray, andpartially remains as an unreacted monomer. This may impair the stabilityof a residual adhesion strength of a pressure-sensitive adhesive afterremoval of the release material.

[0008] On the other hand, Japanese Unexamined Patent Publication (Kokai)Nos. 55-65281, 55-80479, 55-152775 and 55-155079 disclose a method forproducing a pressure-sensitive adhesive tape or sheet where the releaselayer is a release material consisting of a polyolefin-based elastomeror a release material consisting of a mixture of a polyolefin-basedelastomer and polyethylene and the adhesive layer is apressure-sensitive adhesive mainly comprising a polyacrylic ester. Thepolyolefin-based elastomer used in these publications is one having ashear storage modulus according to a JIS K7213 test of 2.0×10⁷ Pa orless and a surface wetting property such that the equilibrium contactangle to a standard solution having a surface tension of 50 mN/m for usein the JIS K6768 test is 55° or more at a temperature of 20±1° C. and arelative humidity of 65±5%. The release material disclosed in thesepublications is formed by extrusion molding and the release materialobtained is not subjected to any treatment. The untreated releasematerial has a problem in that the adhesion between this releasematerial and a substrate supporting the release material is low.Furthermore, the release strength of the release material from thepressure-sensitive adhesive is relatively high and in particular, whenexposed to a high temperature, the release strength is increased and therelease material obtained disadvantageously fails in having asufficiently low release strength.

SUMMARY OF THE INVENTION

[0009] The present invention provides a release material that isenhanced in its anchoring to a substrate, relatively low in its releasestrength from a pressure-sensitive adhesive (even after the exposure toa high temperature), and capable of allowing the pressure-sensitiveadhesive to hold a stable residual adhesion strength. Also disclosed arerelease material articles using the same and a process for producing thearticle.

[0010] Specifically, the present invention provides a release materialformed by irradiating a release material precursor having a shearstorage modulus (hereinafter sometimes referred to as a “storagemodulus” or “G′”) of 1×10² to 3×10⁶ Pa at 20° C. and a frequency of 1Hz, wherein the release material has a contact angle of 15° or more whenmeasured using a mixed solution of methanol and water (volume ratio:90/10) having a wet tension of 25.4 mN/m.

[0011] The release material of the present invention is enhanced in itsanchoring to a substrate by using treatment of radiation on an untreatedrelease material precursor. Furthermore, by virtue of this treatment,the release material of the present invention exhibits relatively lowrelease strength from a pressure-sensitive adhesive, even after theexposure to a high temperature. Still further, after the release of therelease material from a pressure-sensitive adhesive, thepressure-sensitive adhesive can hold a stable residual adhesionstrength.

[0012] The term “contact angle” as used in the present application isdefined as a contact angle measured using a mixed solution of methanoland water (volume ratio: 90/10) having a wet tension of 25.4 mN/mdescribed in JIS K6768:1999. This measurement is conducted at atemperature of 23±1° C. and a relative humidity of 50±5%.

[0013] In present application, the term “release strength from anacrylic pressure-sensitive adhesive,” as used in the evaluation of therelease property of the release material from a pressure-sensitiveadhesive, means a value of release strength measured by the followingmethod. A release material layer is applied to a pressure-sensitiveadhesive sheet consisting of an acrylic pressure-sensitive adhesive(PSA). The PSA is produced by forming a copolymer from a monomerconsisting of butyl acrylate, acrylic acid, vinyl acetate, and2-hydroxyethyl acrylate at a weight ratio of 100:8:2:0.2 and addingisophorone diisocyanate corresponding to 1%, in terms of the mass ratio,to crosslink the copolymer. In this state, the sheet is left standing inan oven at 110° C. for 6 hours. Thereafter, the sheet is left standingovernight at room temperature (25° C.) and used as a sample formeasurement. The release strength of this sample is measured at arelease angle of 180° and a release rate of 300 mm/min at roomtemperature (25° C.). The value, thus obtained, is defined as therelease strength.

[0014] Furthermore, the shear storage modulus (G′) as used in thepresent application is a value measured using a viscoelasticity analyzerat 20° C. and a frequency of 1 Hz.

[0015] In the present application, the terms “(meth)acrylic acid” and“alkyl (meth)acrylate” are used to indicate an acrylic acid or amethacrylic acid, and an alkyl acrylate or an alkyl methacrylate,respectively. In addition, the term “polymer” includes a binarycopolymer, a ternary copolymer, a quaternary or greater copolymer andderivatives thereof. These polymers may also be used as a mixture.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a cross-sectional view showing a release sheet accordingto one embodiment of the present invention.

[0017]FIG. 2 is a cross-sectional view showing one embodiment of aback-treated tape or sheet using the release sheet of the presentinvention.

[0018]FIG. 3 is a cross-sectional view showing one embodiment of apressure-sensitive adhesive tape or sheet with a support using therelease sheet of the present invention.

[0019]FIG. 4 is a cross-sectional view showing one embodiment of apressure-sensitive double-coated tape or sheet using the release sheetof the present invention.

[0020]FIG. 5 is a cross-sectional view showing another embodiment of apressure-sensitive double-coated tape or sheet using the release sheetof the present invention.

[0021]FIG. 6 is a cross-sectional view showing one embodiment of atransfer tape or sheet using the release sheet of the present invention.

[0022]FIG. 7 is a cross-sectional view showing another embodiment of atransfer tape or sheet using the release sheet of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] The present invention is described below by referring to thepreferred embodiments. However, it would be easily understood for oneskilled in the art that the present invention is not limited to thesepreferred embodiments.

[0024] As described above, the release material of the present inventionis formed by irradiation, such by using as an ultraviolet ray orelectron beam, a release material precursor having a shear storagemodulus of 1×10² to 3×10⁶ Pa at 20° C. and a frequency of 1 Hz. Examplesof the release material precursor include polymers having a shearstorage modulus within the above-described range, such as, for example,a poly(meth)acrylic ester, a polyolefin, or a polyvinyl ether.

[0025] One embodiment of the release material precursor according to thepresent invention is a copolymer consisting of two kinds of acrylmonomer components, for example, a (meth)acrylate containing an alkylgroup having from about 12 to about 30 carbon atoms (hereinaftersometimes referred to as a “first alkyl (meth)acrylate”) and a(meth)acrylate containing an alkyl group having from 1 to about 12carbon atoms (hereinafter referred to as a “second alkyl(meth)acrylate”).

[0026] In this case, the first alkyl (meth)acrylate contains arelatively long alkyl side chain having from about 12 to about 30 carbonatoms. The relatively long alkyl side chain has a function of decreasingthe surface energy of the release material. By this function, an effectof relatively preventing the pressure-sensitive adhesive from wettingover the release material surface can be obtained. Accordingly, thefirst alkyl (meth)acrylate acts to impart a low release strength to thefinally produced release material. More specifically, in the contactangle method as an indirect method of estimating the surface energy, therelease material typically has a contact angle of 15° or more.Furthermore, according to the present invention, the first alkyl(meth)acrylate does not contain a polar group on the side chain, such ascarboxyl group (e.g., (meth)acrylic acid), hydroxyl group (e.g.,hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate), or anitrogen- or phosphorus-containing polar group (e.g., (meth)acrylamide,morpholine (meth)acrylate and 2-acryloyloxyethyl acid phosphate).Accordingly, the first alkyl (meth)acrylate can impart relatively lowrelease strength to the release material, not only at low temperatures,but also even after the exposure to high temperatures. In particular, asdescribed later, when the release material precursor is irradiated,using, for example, an electron beam or ultraviolet ray, to form arelease material, the release material can impart a relatively low andconstant release strength of about 0.05 N/25 mm to about 2 N/25 mm.

[0027] Preferred examples of the first alkyl (meth)acrylate having along chain alkyl group include lauryl (meth)acrylate, cetyl(meth)acrylate, octadecyl (meth)acrylate and behenyl (meth)acrylate.Although the content of this first alkyl (meth)acrylate is not limited,the first alkyl (meth)acrylate is suitably contained in an amount ofapproximately 10% to approximately 90% by weight based on the totalamount of the first alkyl (meth)acrylate and the second alkyl(meth)acrylate. If the first alkyl (meth)acrylate content is less thanabout 10% by weight, the release strength may increase. Furthermore, ifthe content exceeds about 90% by weight, non-smooth release, namely,“jerky release” may occur because of high crystallinity thereof.

[0028] The second alkyl (meth)acrylate contains a relatively short alkylside chain having from 1 to about 12 carbon atoms. This relatively shortalkyl side chain decreases the glass transition temperature of therelease material to about 30° C. or less. In turn, the release materialprecursor is reduced in crystallinity and also in the shear storagemodulus to about 1×10² to about 3×10⁶ Pa. As a result, smooth releasecan be attained between a release material article and apressure-sensitive adhesive layer. In the present application, the shearstorage modulus is measured at 20° C. and a frequency of 1 Hz using aviscoelasticity analyzer (Dynamic Analyzer RDAII, manufactured byRheometrics).

[0029] In one embodiment of the present invention, the second alkyl(meth)acrylate containing an alkyl group having 12 carbon atoms may bethe same as the first alkyl (meth)acrylate having the same number ofcarbon atoms. In this case, unless other components are present, therelease material can be formed from a release material precursorcontaining a homopolymer. Furthermore, according to the presentinvention, the second alkyl (meth)acrylate does not contain a polargroup on the side chain, such as a carboxyl group (e.g., (meth)acrylicacid), a hydroxyl group (e.g., hydroxyethyl (meth)acrylate), or anitrogen- or phosphorus-containing polar group (e.g., (meth)acrylamide,morpholine (meth)acrylate and 2-acrloyloxyethyl acid phosphate).Therefore, similarly to the first alkyl (meth)acrylate, the second alkyl(meth)acrylate imparts a relatively low release strength, not only at alow temperature, but also at a high temperature.

[0030] Preferred examples of the second alkyl (meth)acrylate having ashort chain group include butyl (meth)acrylate, hexyl (meth)acrylate,octyl (meth)acrylate, and lauryl (meth)acrylate. The content of thesecond alkyl (meth)acrylate is also, similarly to the first alkyl(meth)acrylate, not limited. However, the second alkyl (meth)acrylate ispreferably contained in an amount of approximately 10% to approximately90% by weight based on the total amount of the first alkyl(meth)acrylate and the second alkyl (meth)acrylate. If the second alkyl(meth)acrylate content is less than about 10% by weight, the property ofthe first alkyl (meth)acrylate surpasses the property of the secondalkyl(meth)acrylate and higher crystallinity and jerky release mayresult. Furthermore, if the content exceeds about 90% by weight, therelease strength tends to become higher.

[0031] The first and second alkyl (meth)acrylates each may be a(meth)acrylate having a branched side chain, such as 2-heptylundecylacrylate, 2-ethylhexyl (meth)acrylate, and isononyl (meth)acrylate. The(meth)acrylate having a branched side chain itself reduces thecrystallinity and therefore, decreases the shear storage modulus and thesurface energy. Therefore, a polymer, other than the copolymerconsisting of a monomer component containing a combination of a firstalkyl (meth)acrylate and a second alkyl (meth)acrylate, may also be usedin some cases. A homopolymer consisting of a monomer component of alkyl(meth)acrylate containing a branched alkyl group having from about 8 toabout 30 carbon atoms is useful as the release material precursor and,for example, a homopolymer of 2-heptylundecyl acrylate preferablyconstitutes a release material precursor from the standpoint that theobtained release material can be reduced in surface energy and shearstorage modulus.

[0032] Preferred release material precursors can be obtained bypolymerization of alkyl (meth)acrylates in the presence of apolymerization initiator. The polymerization initiator is notparticularly limited as long as it can bring about the polymerization,and examples thereof include azobis compounds, such as2,2′-azobisisobutyronitrile, 2,2′-azobis(2-methylbutylonitrile) and2,2-azobis(2,4-dimethylvaleronitrile); and peroxides, such as benzoylperoxide and lauroyl peroxide. For example, 2,2′-azobisisobutyronitrileand 2,2′-azobis(2-methylbutylonitrile) are commercially available undertrade names of V-60 and V-59, respectively, from Wako Pure ChemicalIndustries, Ltd. The content of the polymerization initiator is notlimited as long as a polymer can be formed. However, the polymerizationinitiator is preferably contained in an amount of about 0.005% to about0.5% by weight based on the weight of the monomer. If the polymerizationinitiator content is less than about 0.005% by weight, thepolymerization rate tends to decrease, whereas if the content exceedsabout 0.5% by weight, it tends to result in a lower molecular weight.

[0033] The polymerization of the above-described alkyl (meth)acrylatescan be performed by any known method. However, a solution polymerizationmethod, which involves dissolving the alkyl (meth)acrylates in a solventand polymerizing them in solution is suitably used, because the polymersolution can be directly taken out and used after the completion ofpolymerization. In this case, the solvent is not particularly limited,but examples thereof generally include ethyl acetate, methyl ethylketone, and heptane. From the standpoint of controlling the molecularweight, a chain transfer agent may be suitably incorporated into thesolvent. The solution polymerization of the polymerizable compositionmay be usually performed at a reaction temperature of about 50° C. toabout 100° C. for about 3 to about 24 hours in an atmosphere of an inertgas, such as nitrogen.

[0034] In the case where the release material precursor is apoly(meth)acrylate, the polymer constituting the release materialprecursor suitably has a weight average molecular weight of about100,000 to about 2,000,000. If the weight average molecular weight isless than about 100,000, the release strength may increase, whereas ifthe weight average molecular weight exceeds about 2,000,000, the releasestrength may be satisfactory, but the viscosity of the polymer solutionmay be increased during the synthesis, making handling of the polymersolution difficult.

[0035] As long as the above-described physical properties can beattained, the release material precursor can be constituted by apolyolefin. This polyolefin is formed of an olefin monomer having fromabout 2 to about 12 carbon atoms Examples of the starting monomerinclude linear olefins; such as ethylene, propylene, 1-butene,1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene,1-undecene, 1-dodecene; and branched olefins, such as4-methyl-1-pentene, 5-methyl-1-hexene, 4-methyl-1-hexene,7-methyl-1-octene, and 8-methyl-1-nonene. However, a homopolymer ofethylene or propylene, namely polyethylene and polypropylene, generallycannot satisfy the physical property of shear storage modulus because oftheir crystallinity. Therefore, in the case of using ethylene, propyleneor the like, the shear storage modulus is typically decreased by thecopolymerization, for example, with 1-butene, 1-octene or the like. Withrespect to the copolymer structure, a random copolymer is most preferredfrom the standpoint of reducing the crystallinity. However, even if thecopolymer has crystallinity, as long as the shear storage modulus isacceptable, a block copolymer may also be used. The weight averagemolecular weight is suitably from about 100,000 to about 2,000,000. Ifthe weight average molecular weight is less than about 100,000, therelease strength tends to increase, whereas if the weight averagemolecular weight exceeds about 2,000,000, the release strength may besatisfactory, but the release material precursor, when purified anddried after the polymerization, may not be easily dissolvable again in asolvent. Polyolefins having a high molecular weight can be produced by aconventionally known polymerization methods, for example, ionicpolymerization, preferably coordinated anionic polymerization. Examplesof the commercially available polyolefins include ethylene/propylenecopolymers available under the trade names of EP01P and EP912P from JSR,an ethylene/butene copolymer available under the trade name of EBM3021Pfrom JSR, and an ethylene/octene copolymer available under the tradename of Engage 8407 from DuPont Dow Elastomers.

[0036] The release material precursor may also be a polyvinyl etherhaving the above-described properties. Examples of the starting monomerof this polyvinyl ether include linear or branched vinyl ethers such asn-butyl vinyl ether, 2-ethylhexyl vinyl ether, dodecyl vinyl ether, andoctadecyl vinyl ether. However, for example, polyoctadecyl vinyl etherdoes not satisfy the above-described physical property for the shearstorage modulus and accordingly, in the case of using octadecyl vinylether, the shear storage modulus is typically decreased by thecopolymerization, for example, with 2-ethylhexyl vinyl ether. Withrespect to the copolymer structure, random copolymer is most preferredfrom the standpoint of reducing the crystallinity. However, even if thecopolymer has crystallinity, as long as the shear storage modulus isacceptable, a block copolymer may also be used. The weight averagemolecular weight is suitably from about 100,000 to about 2,000,000. Ifthe weight average molecular weight is less than about 100,000, therelease strength tends to increase, whereas if the weight averagemolecular weight exceeds about 2,000,000, the release strength may besatisfactory, but the viscosity of the polymer solution may be increasedduring the synthesis such that handling of the polymer solution tends tobe difficult. The polyvinyl ether can be produced by ionicpolymerization, for example, by cationic polymerization.

[0037] The release material precursor is provided on a substrate,preferably consisting of polyester, polyolefin, or paper, and subjectedto a treatment of radiation, such as using an electron beam orultraviolet rays, whereby a release material article can be provided inthe form of a tape or sheet. In this case, the release materialprecursor generally has no polar functional groups, such as carboxylgroups, hydroxyl groups, or amide groups, as described above, andtherefore, the release material precursor itself is expected to havepoor anchoring to the substrate. However, in the present invention, ithas been found that, despite the absence of a polar functional group inthe release material precursor, the anchoring between the substrate andthe release material can be elevated by treatment with radiation and, atthe same time, the release strength between the resulting releasematerial and a pressure-sensitive adhesive can be maintained at arelatively low value of about 0.05 to about 2 N/25 mm.

[0038] This release material article is manufactured as follows. Asolution of the release material precursor is diluted with a diluentcontaining at least one of ethyl acetate, butyl acetate, methyl ethylketone, methyl isobutyl ketone, hexane, heptane, toluene, xylene, andmethylene chloride, and then coated on a substrate to a predeterminedthickness, thereby forming a release material precursor layer on thesubstrate. In this case, the diluting solvent may be the same as ordifferent from the solvent used in the solution polymerization. Thesediluents are inexpensive and readily available as compared withfluorine-containing solvents used for a fluorine-type release materialand therefore, the cost of materials can be reduced in the presentinvention. Examples of the substrates that can be used include, forexample, plastics such as polyesters and polyolefins, and paper. Thesubstrate generally used is a polyester (e.g., a polyethyleneterephthalate, polyethylene naphthalate, or polybutylene terephthalatefilm). The thickness of the release material precursor depends on thekind of the substrate but is generally from about 0.01 to about 1 μm,preferably from about 0.05 to about 0.5 μm. If the thickness is lessthan about 0.01 μm, a uniform release layer may be difficult to form andthe release strength may increase, whereas even if the thickness exceedsabout 1 μm, the release strength would not be significantly changed.

[0039] The release material precursor is irradiated by, for example, anelectron beam or ultraviolet ray. In the case of using an electron beam,the irradiation is generally performed under an inert gas, such asnitrogen. The absorbed dose thereto depends on the thickness orcomposition of the release material precursor layer and is usually from1 to about 100 kGy. If the absorbed dose is less than about 1 kGy, theanchoring between the substrate and the release material may not beincreased. Furthermore, if the absorbed dose exceeds about 100 kGy, aside reaction such as surface oxidation may take place to produce apolar functional group. The production of a polar functional group maygive rise to deterioration of the substrate.

[0040] If an ultraviolet ray is used in place of an electron beam, theabove-described inert gas is not necessary. The irradiation energy ofthe ultraviolet ray depends on the thickness or composition of therelease material precursor layer and is usually from about 10 to about300 mJ/cm², preferably from about 20 to about 150 mJ/cm². If the energyis less than about 10 mJ/cm², a sufficiently high anchoring to thesubstrate may not be attained and, if the irradiation energy exceedsabout 300 mJ/cm², the release strength may increase.

[0041] It is revealed that irradiation, such as with an electron beam orultraviolet ray, increases the anchoring between the substrate and therelease material and decreases the release strength between the releasematerial and the pressure-sensitive adhesive. Improving the anchoringbetween the substrate and the release material by irradiation can becompleted within a very short time, as compared with conventionalmethods where a crosslinking agent and a release material having anactive group capable of reacting with the crosslinking agent arecrosslinked by adding a thermal energy or the like to improve theanchoring. As a result, the production rate of the release materialarticle increases. Furthermore, in the present invention, a crosslinkingagent is not necessary and therefore, remaining of a crosslinking agentby itself in the release material does not occur, so that a so-called“aging” or “ripening” process until the stabilization of performance asrequired in conventional release material articles is not necessary.Still further, by the irradiation of a radiation, the release materialis inhibited from transferring to the pressure-sensitive adhesive layerand this brings about an effect that, even after the exposure to a hightemperature, the low release strength between the release material andthe pressure-sensitive adhesive can be maintained.

[0042] The kind of the pressure-sensitive adhesive is not particularlylimited. However, when an acrylic pressure-sensitive adhesive including(meth)acrylic resin is used as a pressure-sensitive adhesive, therelease material of the present invention can particularly work well inthat the release strength for the pressure-sensitive adhesive is low.

[0043]FIG. 1 shows a cross-sectional view of a release material article(release sheet (1)) comprising a substrate (3) and a release material(2). This release material article can be used in various types ofpressure-sensitive adhesive tapes or sheets.

[0044]FIG. 2 is a cross-sectional view of a back-treated tape or sheetwhere a pressure-sensitive adhesive layer (4) is formed on the substrate(3) of a release sheet (1). In this type of tape or sheet, thepressure-sensitive adhesive layer is stacked on the release materiallayer of a tape or sheet having the same structure.

[0045]FIG. 3 is a cross-sectional view of a pressure-sensitive adhesivetape or sheet where a pressure-sensitive adhesive layer (4) is supportedby a support (5).

[0046]FIGS. 4 and 5 are cross-sectional views of a pressure-sensitiveadhesive double-coated tape or sheet.

[0047]FIGS. 6 and 7 are cross-sectional views of a transfer tape orsheet.

[0048] In the foregoing part, the present invention is described byreferring to preferred embodiments. However, the present invention isnot limited thereto. In the case where the release material precursor isa poly(meth)acrylate, this polymer may be constructed by a plurality ofmonomers and, for example, the first alkyl (meth)acrylate and the secondalkyl (meth)acrylate each may be constructed by a plurality of monomers.The monomer component for the polymer constituting the release materialprecursor in principal does not contain a polar functional group, but aslong as the shear storage modulus and the contact angle satisfy theabove-described ranges, at least one polar functional group may becontained in the side chain.

[0049] Furthermore, the release material precursor is not limited tothose copolymers and homopolymers consisting of an alkyl (meth)acrylate.The polymer constituting the release material precursor is notparticularly limited as long as the polymer has a shear storage modulusand a contact angle within the above-described range. The polymer may bea polyolefin, which is constituted by a monomer component such asethylene, propylene, and octene, or a polyvinyl ether, such as onesformed of octadecyl vinyl ether and 2-ethylhexyl vinyl ether. Therelease material precursor containing such a polymer can haveparticularly excellent insulating property, weather resistance andsolvent resistance.

EXAMPLES

[0050] The present invention is described below by referring to theExamples. However, it is easily understood for one skilled in the artthat the present invention is not limited thereto. In the following, the“parts” is “parts by weight” in all cases.

Example 1

[0051] 1. Preparation of Release Material Precursor

[0052] In a reactor vessel, 7.2 parts of octadecyl acrylate, 4.8 partsof butyl acrylate and 28 parts of ethyl acetate were uniformly mixed toprepare a solution. To the reaction vessel, 0.004 parts of an initiatorof 2,2-azobis(2,4-dimethylvaleronitrile) was further added. Thisinitiator is commercially available under the trade name of V-65 fromWako Pure Chemical Industries, Ltd.

[0053] Thereafter, the reactor was purged with nitrogen gas for 10minutes and then this reactor was transferred to a rotating constanttemperature bath at 50° C. and left standing there for 20 hours. It wasfound that octadecyl acrylate reacted with butyl acrylate in the vesselto produce a release material precursor consisting of an acryliccopolymer within the solution. Thereafter, the weight average molecularweight of this release material precursor was measured by gel permeationchromatography (GPC). In the measurement, a liquid chromatograph (1090SERIES II) manufactured by Hewlett Packard was used. As a result of themeasurement, the release material precursor of this Example was found tohave a weight average molecular weight of 800,000 on the basis ofpolystyrene standard. Also, the shear storage modulus of the releasematerial precursor was measured at 20° C. and a frequency of 1 Hz usinga dynamic viscoelasticity analyzer (Dynamic Analyzer RDAII, manufacturedby Rheometrics). As a result, the shear storage modulus was found to be8.5×10⁵ Pa.

[0054] 2. Manufacture of Release Sheet

[0055] The release material precursor prepared above was diluted withethyl acetate without taking the precursor out of the solution, toprepare a diluted solution having a solid content of 1 wt %. Thisdiluted solution was coated by a bar coater on a polyethyleneterephthalate (PET) substrate having a thickness of 50 μm and then theethyl acetate was evaporated to form a release material precursor layerhaving a thickness of about 0.1 μm. The resulting sheet was overlaid ona web moving at a web speed of 30 m/min and thereon, an electron beamhaving a constant adsorbed dose and accelerated by a voltage of 200 kVwas irradiated to obtain a release sheet. In this Example, an electronbeam having an adsorbed dose of 0 (non-irradiation), 10, 30, 50 or 70kGy was irradiated to manufacture release sheets.

[0056] 3. Evaluation of Release Sheet

[0057] For each release sheet obtained above, a release strength of therelease sheet from the pressure-sensitive adhesive described below, aresidual adhesion strength of the pressure-sensitive adhesive afterremoval of the release sheet and a contact angle of the release sheetwith the solution described below were measured and evaluated asfollows. On the release sheet, an acrylic pressure-sensitive adhesivesheet (a copolymer formed of monomers containing butyl acrylate, acrylicacid, vinyl acetate and 2-hydroxyethyl acrylate at a weight ratio of100:8:2:0.2 was crosslinked by adding isophorone diisocyanatecorresponding to 1% by weight of the copolymer) was stacked and adhered.The sheet in this state was left standing in an oven at 110° C. for 6hours. Thereafter, the release sheet was taken out of the oven togetherwith the acrylic pressure-sensitive adhesive sheet and left standingovernight at room temperature.

[0058] The release sheet was continuously peeled off from the acrylicpressure-sensitive adhesive sheet using an autograph (AGS-100B)manufactured by Shimadzu Corp., so that the load per 25 mm in width wasmeasured as a release strength. In this measurement, the release anglewas 180° and the release speed was 300 mm/min.

[0059] The resulting pressure-sensitive adhesive sheet was adhered to astainless steel plate and then pressed by a roller of 2 kg. In thispressing, the roller was once reciprocated on the pressure-sensitiveadhesive sheet at a speed of 300 mm/min. After the pressing, when 20minutes was passed, the adhesive strength was measured as a residualadhesion strength. In this measurement, the release angle was 180° andthe release speed was 300 min/min.

[0060] Separately, the release sheet obtained above was measured on thecontact angle by a contact angle meter CA-A manufactured by Kyowa KagakuK.K. In the measurement, a mixed solution of methanol and water (90/10by volume) having a wet tension of 25.4 mN/m as described inJIS6768:1999 was used. This measurement was performed within 5 secondsafter the dripping.

[0061] The release strength of each release sheet, the residual adhesionstrength of each pressure-sensitive adhesive and the contact angle ofeach release sheet are shown in Table 1. As apparently seen from Table1, the release strength was high whenever the adsorbed dose was toosmall or too large. Particularly, the sample not irradiated with anelectron beam lacked the anchoring between the substrate and the releasematerial layer and therefore, the release material transferred into thepressure-sensitive adhesive layer during aging, causing it to bedifficult to release from the adhesive layer. Accompanying thistransfer, the pressure-sensitive adhesive and the substrate of therelease sheet were stuck together and, upon a release strength test,cohesive failure of the pressure-sensitive adhesive layer was observed.From this, it is seen that by irradiating the release material precursorof the present invention with an electron beam, the release sheetcomprising a release material having low release strength for apressure-sensitive adhesive and satisfactorily anchored to the substratecan be formed. Furthermore, when the release sheet not irradiated withan electron beam is compared with the release sheet irradiated with anelectron beam having an adsorbed dose of 70 kGy, the contact angle issmaller in the release sheet irradiated with an electron beam. This isconsidered to result because a polar group was formed on the releasesheet surface by the side reaction such as surface oxidation. From theseresults, it is seen that when the adsorbed dose was 30 kGy, smoothrelease with lowest release strength was realized and the obtainedrelease sheet maintained sufficiently high residual adhesion strength.However, it is noted that the adsorbed dose for obtaining optimalrelease strength and sufficiently high residual adhesion strength variesdepending on the mixture ratio of the monomers or coating thickness.TABLE 1 Residual Adhesion Absorbed Dose Release Strength StrengthContact Angle (kGy) (N/25 mm) (N/25 mm) (°) 0 10.78 — 38.7 6 11.27 7.9937.6 10 5.60 9.02 37.6 30 0.34 9.31 36.6 50 0.64 9.51 35.3 70 2.60 9.3129.1

Example 2

[0062] The preparation of release material precursor, the production ofrelease sheet and the evaluation of release sheet were performed inaccordance with Example 1 except that 4.8 parts of octadecyl acrylateand 7.2 parts of 2-ethylhexyl acrylate were used in this Example inplace of using 7.2 parts of octadecyl acrylate and 4.8 parts of butylacrylate. The release material precursor of this example had a shearstorage modulus of 7.2×10⁴ Pa. The release strength of the releasesheet, the residual adhesion strength of the pressure-sensitive adhesiveand the contact angle of the release sheet are shown in Table 2. It isseen that in the case of the composition of this Example, smooth releasewith lowest release strength was realized and at the same time, asufficiently high residual adhesion strength was maintained when theadsorbed dose was 10 kGy. TABLE 2 Residual Adhesion Absorbed DoseRelease Strength Strength Contact Angle (kGy) (N/25 mm) (N/25 mm) (°) 60.91 9.60 27.9 10 0.27 10.09 27.4 30 0.54 8.97 26.4 50 2.21 6.96 24.1 702.99 4.95 21.5

Comparative Example 1

[0063] A release material precursor was prepared in accordance withExample 1 except that 12.0 parts of octadecyl acrylate was used in thisExample in place of using 7.2 parts of octadecyl acrylate and 4.8 partsof butyl acrylate. The production of release sheet and the evaluation ofrelease sheet were performed in accordance with Example 1 except thatthe release material precursor was diluted with a mixed solvent of ethylacetate and n-heptane (50/50 by weight) in place of ethyl acetate. Therelease material precursor of this Example had a shear storage 5×10⁷ Pa.The release strength of the release sheet, the residual adhesionstrength of the pressure-sensitive adhesive and the contact angle of therelease sheet are shown in Table 3. As is apparently seen from Table 3,in the case of octadecyl acrylate homopolymer, the contact angle waslarge as compared with Example 1 or 2 where octadecyl acrylate wascopolymerized with butyl acrylate or 2-ethylhexyl acrylate. When thecontact angle is large, it is expected that the pressure-sensitiveadhesive is prevented from wet spreading and the release strength isdecreased. However, even if the conditions at the irradiation ofelectron beam was varied, the release strength was high. Furthermore, inthe case of the release material, jerky release was observed due tocrystallinity. In particular, although a release material having arelatively low release strength was produced with a relatively lowabsorbed dose of 50 kGy or 70 kGy, smooth release could not be attained.The above-described relatively high modulus (i.e., 1.5×10⁷ Pa)considered to be caused by this high crystallinity. TABLE 3 ResidualAdhesion Absorbed Dose Release Strength Strength Contact Angle (kGy)(N/25 mm) (N/25 mm) (°) 6 12.25 6.62 50.2 10 11.27 6.37 49.3 30 8.185.15 48.2 50 2.99 10.01 48.6 70 2.40 10.05 45.4 90 5.39 6.62 45.9 1107.25 5.39 42.8

Comparative Example 2

[0064] The preparation of release material precursor, the production ofrelease sheet and the evaluation of release sheet were performed inaccordance with Example 1 except that 12.0 parts of butyl acrylate wasused in this Example in place of using 7.2 parts of octadecyl acrylateand 4.8 parts of butyl acrylate. The release material precursor of thisExample had a shear storage modulus of 3.1×10⁴ Pa. The release strengthof the release sheet, the residual adhesion strength of thepressure-sensitive adhesive and the contact angle of the release sheetare shown in Table 4. In the case of butyl acrylate homopolymer, incontrast with the octadecyl acrylate homopolymer in Comparative Example1, the as very small. Using this, release sheets were manufactured byvarying conditions for the irradiation of electron beam but all revealedhigh.

[0065] The contact angle of the release sheet was measured using a mixedsolution of methanol and water (90/10 by volume) having a wet tension of25.4 mN/m described in JIS K6768: 1999 mentioned above. The wet tensionof 25.4 mN/m is a relatively low value for the solution used in the wettension test. This solution was used for the measurement of the contactangle of the release sheet, because a great difference is expected toresult among the contact angle values of various release sheets. In thecase where a solution having a large wet tension, for example, water(wet tension: 73.0 mN/m) was used in the measurement of the the contactangle was almost 110° in all release sheets and it was difficult to findsignificant differences among them. TABLE 4 Residual Adhesion AbsorbedDose Release Strength Strength Contact Angle (kGy) (N/25 mm) (N/25 mm)(°) 3 3.77 8.09 12.2 4.5 3.82 7.84 11.3 6 3.68 7.79 11.5 10 3.72 7.8910.7 30 5.68 7.60 10.8 50 6.62 8.33 10.1 70 7.60 8.00 8.2

Comparative Example 3

[0066] The preparation of release material precursor, the production ofrelease sheet and the evaluation of release sheet were performed inaccordance with Example 1 except that the release material precursor wasformed from an octadecyl acrylate/acrylic acid copolymer (molar ratio ofmonomers: 60/40) which is commonly used as a back surface treatingagent. The release strength of each release sheet of this Example isshown in Table 5. In the case of coating the octadecyl acrylate/acrylicacid copolymer, cohesive failure of the pressure-sensitive adhesive wasobserved not only in the sample non-irradiated with electron beam butalso in all samples and thus, the samples were verified not to serve asa release sheet without measuring the contact angle and the shearstorage modulus. This is caused because the release material containedacrylic acid, which is an acrylic component having high polarity. Theaffinity for the pressure-sensitive adhesive was, therefore, increasedduring the aging at a high temperature. Due to the occurrence ofcohesive failure in the pressure-sensitive adhesive, the residualadhesion strength could not be measured. TABLE 5 Absorbed Dose ReleaseStrength (kGy) (N/25 mm) 0 20.97 6 20.09 10 22.34 30 19.40 50 20.68 7020.45

Example 3

[0067] The preparation of release material precursor, the production ofrelease sheet and the evaluation of release sheet were performed inaccordance with Example 1 except that a constant energy amount ofultraviolet ray was irradiated in this Example in place of irradiatingelectron beam on the release material precursor layer. In this Example,the ultraviolet ray was irradiated using a high-pressure mercury lamp(Model H bulb) manufactured by Fusion System Corporation. Also, byirradiating ultraviolet ray having an energy of 0 (non-irradiated), 10,20, 40, 80 or 20, respective release sheets were manufactured. Therelease strength of the release sheet, the residual adhesion strength ofthe pressure-sensitive adhesive and the contact angle of the releasesheet are shown in Table 6. As is apparently seen from the Table, therelease strength was high whichever the irradiation energy was too smallor too large. Particularly, the sample not irradiated with ultravioletray lacked the anchoring between the substrate and the release materiallayer and therefore, the release material was transferred into thepressure-sensitive adhesive layer during aging to cause increase in therelease strength. Accompanying this transfer into the pressure-sensitiveadhesive, the pressure-sensitive adhesive and the substrate of therelease sheet were stuck together and upon a release strength test, acohesive failure was observed in the pressure-sensitive adhesive layer.It is also seen that when the irradiation energy is too large, a sidereaction such as surface oxidation takes place and the release strengthbecomes high. In other words, by irradiating an appropriate does ofultraviolet ray on the coated release material precursor, the releasesheet which has anchoring of the release material to the substrate andat the same time, realize smooth release with low release strength, canbe formed. TABLE 6 Irradiation Energy Release Strength Residual AdhesionContact Angle (mJ/cm²) (N/25 mm) Strength (N/25 mm) (°) 0 10.58 — 38.710 5.15 10.19 38.8 20 0.25 9.60 38.8 40 0.29 10.44 37.9 80 0.35 10.2937.4 200 0.80 10.05 34.5

Example 4

[0068] A release material precursor was prepared in the same manner asin Example 3 except that 6.0 parts of octadecyl acrylate and 6.0 partsof butyl acrylate were used, namely, the weight ratio of octadecylacrylate and butyl acrylate was changed to 50/50, in this Example inplace of using 7.2 parts of octadecyl acrylate and 4.8 parts of butylacrylate. From this release material precursor, a release sheet wasproduced in the same manner as in Example 3 except that the ultravioletirradiation dose was fixed to 40 mJ/cm². The release material precursorof this Example had a shear storage modulus of 4.4×10⁵ Pa.

Example 5

[0069] The preparation of release material precursor and the productionof release sheet were performed in the same manner as in Example 4except that 4.8 parts of octadecyl acrylate and 7.2 parts of butylacrylate were used, namely, the weight ratio of octadecyl acrylate andbutyl acrylate was changed to 40/60, in this Example in place of using6.0 parts of octadecyl acrylate and 6.0 parts of butyl acrylate. Therelease material precursor of this Example had a shear storage modulusof 1.3×10⁵ Pa.

Comparative Example 4

[0070] The preparation of release material precursor and the productionof release sheet were performed in the same manner as in ComparativeExample 1 except that an ultraviolet ray having an energy amount of 40mJ/cm² was irradiated in this Example in place of irradiating electronbeam on the release material precursor layer.

Comparative Example 5

[0071] The preparation of release material precursor and the productionof release sheet were performed in the same manner as in ComparativeExample 2 except that an ultraviolet ray having an energy amount of 40mJ/cm² was irradiated in this Example in place of irradiating electronbeam on the release material precursor layer.

[0072] The release sheets of Examples 4 and 5 and Comparative Examples 4and 5 were measured and evaluated on the release strength, the residualadhesion strength and the contact angle in the same manner as in Example1 except for using an acrylic pressure-sensitive adhesive sheet (acopolymer formed of a monomer containing butyl acrylate, acrylic acidand vinyl acetate at a weight ratio of 10:8:2 and crosslinked by addingisophorone diisocyanate corresponding to 1.3% of the copolymer) havingan adhesive strength of about 25 N/25 mm to a stainless steel plate inplace of the acrylic pressure-sensitive adhesive having an adhesivestrength of about 10 N/25 mm.

[0073] The release strength of each release sheet, the residual adhesionstrength of each pressure-sensitive adhesive and the contact angle ofeach release sheet, of Examples 4 and 5 and Comparative Examples 4 and 5are shown in Table 7. As seen from Tables 6 and 7, the release sheet ofComparative Example 4 has a contact angle higher than that of Examples 3to 5. From this, the release sheet of Comparative Example 4 is expectedto prevent the wet spreading of the pressure-sensitive adhesive andreduce the release strength. However, the release material precursor hasa shear storage modulus higher than those of Examples 3 to 5 due to itscrystallinity. As a result, as apparently seen from Table 7, the releasestrength thereof is disadvantageously very high.

[0074] It was revealed that the release material precursor ofComparative Example 5 has a shear storage modulus on the same level asthat of Examples 3 to 5, whereas the release sheet of ComparativeExample 5 has a contact angle lower than that of Examples 3 to 5. Asapparently seen from Table 7, this sheet disadvantageously has very highrelease strength, similarly to the release sheet of Comparative Example4. TABLE 7 ODA/BA Residual (octadecyl Adhesion Contact acrylate/butylRelease Strength Strength Angle Example acrylate) (N/25 mm) (N/25 mm)(°) Comparative 100/0  1.7 21.3 50.5 Example 4 Example 4 50/50 0.28 23.723.7 Example 5 40/60 0.38 24.0 20.5 Comparative  0/100 2.5 22.6 11.8Example 5

Example 6

[0075] A release material precursor sheet was prepared in accordancewith Example 1 except that ultraviolet ray was irradiated in a nitrogenatmosphere reduced to an oxygen concentration of 30 ppm or less in placeof irradiating ultraviolet ray on the PET substrate immediately afterthe formation of the release material precursor layer. The productionand the evaluation of release sheet were performed in accordance withExample 3. To speak more specifically, a release material precursorlayer was provided on a PET substrate and this release materialprecursor layer together with the substrate was placed in a quartzglass-made sealed container. Into the container, nitrogen gas wasintroduced to perform the purging until the oxygen concentration became30 ppm or less and immediately, a constant energy amount of ultravioletray was irradiated. The irradiated energy amount was 40, 80 or 200mJ/cm². The results of tests on release strength of each release sheetand the residual adhesion strength of each pressure-sensitive adhesiveare shown in Table 8. On comparison with the results in Table 6 forultraviolet irradiation in air, it is seen that the level of releasestrength and residual adhesion strength under the conditions of givinglowest release is almost the same whichever in air or in nitrogenatmosphere the ultraviolet ray was irradiated. On the other hand, whenexcess ultraviolet ray is irradiated, the release strength is increaseddue to the surface oxidation as in the case of the irradiation in airbut the degree of increase is relatively small in the sheet irradiatedin a nitrogen atmosphere. TABLE 8 Irradiation Energy Release StrengthResidual Adhesion (mJ/cm²) (N/25 mm) Strength (N/25 mm) 40 0.28 10.17 800.33 10.03 200 0.67 10.09

Example 7

[0076] A release material precursor was prepared in accordance withExample 1 except that 12 parts of lauryl methacrylate containing analkyl group having 12 carbon atoms was used in this Example in place ofusing 7.2 parts of octadecyl acrylate and 4.8 parts of butyl acrylate.In this case, the release material precursor had a shear storage modulusof 8.6×10⁵ Pa. Using this release material precursor, the production andthe evaluation of release sheet were performed in accordance withExample 3. The release strength of each release sheet, the residualadhesion strength of each pressure-sensitive adhesive and the contactangle of each release sheet are shown in Table 9. As is apparently seenfrom the Table, in the case of non-irradiation or excessively smallirradiation energy, the anchoring between the substrate and the releasematerial layer was weak, and therefore the release material wastransferred into the pressure-sensitive adhesive layer. As a result, thepressure-sensitive adhesive and the substrate of the release sheet werestuck together and the release strength became high. On the other hand,in the case of irradiation of ultraviolet ray in a sufficiently highamount, the obtained release sheet had low release strength of no morethan 2N/25 mm. Also, with the release material of this Example, it wasseen that jerky release was not observed and smooth release could beobtained. TABLE 9 Residual Irradiation Release Adhesion Energy StrengthStrength Contact Angle (mJ/cm²) (N/25 mm) (N/25 mm) (°) 0 17.80 — 31.810 10.75 8.58 31.5 20 10.95 9.05 31.6 40 1.59 11.26 31.0 80 1.38 11.4530.2 120 1.33 9.82 28.1

Example 8

[0077] A release material precursor was prepared in accordance withExample 1 except that 20 parts of 2-heptylundecyl acrylate dissolved in20 parts of the above-described ethyl acetate/n-heptane mixed solventwas used in this Example in place of using 7.2 parts of octadecylacrylate and 4.8 parts of butyl acrylate. In this case, the releasematerial precursor had a shear storage modulus of 1.2×10³ Pa. Using thisrelease material precursor, the production and the evaluation of releasesheet were performed in accordance with Example 3 except that an ethylacetate/n-heptane mixed solvent was used to dilute the solution. Therelease strength of the release sheet, the residual adhesion strength ofthe pressure-sensitive adhesive and the contact angle of the releasesheet, at an irradiation energy of 100 mJ/cm², are shown in Table 10.

[0078] As is apparently seen from Table 10, the release sheet of thisExample comprises a polymer consisting of 2-heptylundecyl acrylatecontaining an alkyl group having 18 carbon atoms on the side chain.Nevertheless, the release strength thereof is low as compared with thatof Examples 4 and 5 where the release sheet comprises a polymerpartially consisting of octadecyl acrylate having the same carbon atomnumber of 18. Accordingly, it was found that a desirable release sheetcan be obtained by using a homopolymer of a branched alkyl(meth)acrylateas in this Example. This makes a striking contrast to the releasematerial (Comparative Example 4) consisting of octadecyl acrylatehomopolymer, which exhibits high shear storage modulus at roomtemperature due to the crystallization of the side chain. TABLE 10Release Strength Residual Adhesion Contact Example (N/25 mm) Strength(N/25 mm) Angle (°) Example 8 0.16 23.3 38.2

Example 9

[0079] The release material precursor of this Example was prepared inthe same manner as in Example 8 from 2-heptylundecyl acrylate andisobornyl acrylate according to the blend ratio (weight ratio) shown inTable 11. Using this release material precursor, the production and theevaluation of release sheet were performed in accordance with Example 8.The release strength of release sheet, the residual adhesion strength ofpressure-sensitive adhesive and the contact angle of release sheet areshown in Table 11.

[0080] As seen from Table 11, with the increase in the ratio ofisobornyl acrylate, the shear storage modulus elevates. This occursbecause the isobornyl acrylate has a high glass transition point. Inturn, as apparent from the Table, the release strength also increases.TABLE 11 2-HUDA/IBXA Residual Shear (2-heptylundecyl Release AdhesionStorage acrylate/ Strength Strength Contact Modulus isobornyl acrylate(N/25 mm) (N/25 mm) Angle (°) (Pa) 85/15 0.23 24.5 34.6 3.3 × 10³ 75/250.40 25.7 31.0 7.8 × 10³ 50/50 13.3 23.0 26.7 4.1 × 10⁶

Example 10

[0081] The release material precursor of this Example was prepared inthe same manner as in Example 8 from 2-heptylundecyl acrylate and2-ethylhexyl acrylate according to the blend ratio (weight ratio) shownin Table 12. Using this release material precursor, the production andthe evaluation of release sheet were performed in accordance withExample 8. The release strength of release sheet of this Example, theresidual adhesion strength of pressure-sensitive adhesive and thecontact angle of release sheet are shown in Table 12.

[0082] As seen from Table 12, whatever the ratio of 2-ethylhexylacrylate is varied, the shear storage modulus is almost constant. Thisoccurs because the 2-ethylhexyl acrylate has a very low glass transitionpoint. In turn, as apparent from the Table, low release strength isconstantly maintained. TABLE 12 2-HUDA/2-EHA Residual Shear(2-heptylundecyl Release Adhesion Storage acrylate/2- Strength StrengthContact Modululs ethylhexyl acrylate (N/25 mm) (N/25 mm) Angle (°) (Pa)85/15 0.16 21.4 35.6 1.8 × 10³ 75/25 0.16 21.8 34.4 4.7 × 10³ 50/50 0.2225.2 32.4 6.2 × 10³

Example 11

[0083] A release sheet was produced in the same manner as in Example 3except that the substrate on which the release material precursor iscoated was biaxially oriented polypropylene (subjected to coronatreatment) in this Example. The release strength of release sheet, theresidual adhesion strength of pressure-sensitive adhesive and thecontact angle of release sheet are shown in Table 12. As apparently seenfrom the Table, similarly to Example 3, the release strength is highwhichever too small or too large the irradiation energy is. On the otherhand, when an appropriate amount of ultraviolet ray is irradiated on thecoated release material precursor, even if the substrate is changed topolypropylene, the release material anchors the substrate and therelease sheet obtained exhibits low release strength and realizes smoothrelease. For the purpose of providing lower release strength, it can besaid that polyethylene terephthalate is more excellent as the substrate.TABLE 13 Irradiation Energy Release Strength Residual Adhesion Contact(mJ/cm²) (N/25 mm) Strength (N/25 mm) Angle (°)  0 7.06 9.20 38.9 104.41 9.45 38.1 20 2.21 10.31 34.9 40 0.83 10.05 36.4 80 0.96 9.55 37.6200  1.22 9.40 33.6

Example 12

[0084] In this example, an ethylene/propylene copolymer comprising theethylene component of 75% by weight (EP01P, produced by JSR) was used asthe release material precursor. The shear storage modulus of the releasematerial precursor of this Example was determined according to themeasuring method described above and found to be 4.5×10⁵ Pa. Then, thisethylene/propylene copolymer was dissolved in toluene/methyl ethylketone mixed solvent (50/50 by weight) to prepare a solution of releasematerial precursor and thereafter, ultraviolet ray having an irradiationenergy of 0 (non-irradiation), 20, 40, 80, 120 or 400 mJ/cm² asirradiated in the same manner as in Example 3 to prepare release sheets.The release sheets obtained were evaluated using the same acrylicpressure-sensitive adhesive sheet as used in Example 4. The shearstorage modulus of release material precursor, the release strength ofrelease sheet, the residual adhesion strength of pressure-sensitiveadhesive and the contact angle of release sheet are shown in Table 14.As apparently seen from the Table, the release strength is highwhichever too small or too large the irradiation energy is.Particularly, the sample not irradiated with ultraviolet ray lacked theanchoring between the substrate and the release material and therefore,the release material was transferred into the pressure-sensitiveadhesive layer during aging to cause increase in the release strength.Furthermore, the pressure-sensitive adhesive and the substrate of therelease sheet were stuck together and upon a release strength test,cohesive failure was observed in the pressure-sensitive adhesive layer.In addition, it is seen that if the irradiation energy is too large, aside reaction such as surface oxidation takes place and the releasestrength elevates. From this Example, a release sheet which hasanchoring to a substrate and realize smooth release with low releasestrength can be formed by coating the release material precursor andirradiating an appropriate dose of ultraviolet ray thereon, using notonly poly(meth)acrylate but also polyolefin such as ethylene.propylenecopolymer. TABLE 14 Irradiation Release Strength Residual AdhesionContact Energy (mJ/cm²) (N/25 mm) Strength (N/25 mm) Angle (°)  0 33.3 —29.9 20 18.5 16.8 28.9 40 1.41 27.8 27.8 80 0.22 25.7 26.2 120  0.2424.6 24.9 400  0.35 25.5 25.6

Example 13

[0085] In this Example, the production and evaluation of release sheetwere performed, where two kinds of ethylene/propylene copolymers eachhaving almost the same weight ratio of ethylene component as in Example12 were used as the release material precursor and ultraviolet rayhaving a dose of 80 mJ/cm² was irradiated in accordance with Example 12.The release strength of release sheet of this Example, the residualadhesion strength of pressure-sensitive adhesive, the contact angle ofrelease sheet, the weight ratio of ethylene component (C2 component) andthe shear storage modulus of the release material shown in Table 15.

[0086] It is seen from this Table that when the weight ratio of ethylenecomponent is almost the same, as the shear storage modulus of therelease material precursor increases, the release strength of therelease sheet also increases. Particularly, the release materialprecursor consisting of an ethylene/propylene copolymer commerciallyavailable under the trade name of EP961SP from JSR has a high shearstorage modulus and, therefore, imparts high release strength to therelease sheet. On the other hand, in the case of EP912P, the shearstorage modulus is low and, therefore, the release sheet is impartedwith low release strength. TABLE 15 Residual Shear Release AdhesionContact C2 Com- Storage Trade Strength Strength Angle ponent ModulusName^((a)) (N/25 mm) (N/25 mm) (°) (%) (Pa) EP912P 0.17 22.5 26.6 75 2.2× 10⁵ EP961SP 4.3 23.9 28.3 74 3.4 × 10⁶

Example 14

[0087] In this Example, an ethylene/octene copolymer (Engage 8407,produced by Du Pont Dow Elastomers) and an ethylene/butene copolymer(EBM3021P, produced by JSR) each was used as the release materialprecursor and the shear storage modulus thereof was measured by themethod described above. Thereafter, each release material precursor wasdissolved in a toluene/n-heptane mixed solvent (50/50 by weight) toprepare a release material precursor solution. Using these, releasesheets were manufactured and evaluated in the same manner as in Example4. The shear storage modulus of the release material precursor, therelease strength of the release sheet, the residual adhesion strength ofthe pressure-sensitive adhesive, and the contact angle of the releasesheet are shown in Table 16.

[0088] It is seen from this Table that two release material precursorshave a shear storage modulus on the same level as the ethylene/propylenecopolymer (EP01P) of Example 12 and, similarly, each gives an excellentrelease sheet capable of exhibiting low release strength. TABLE 16Residual Shear Release Adhesion Storage Strength Strength ContactModulus Material (N/25 mm) (N/25 mm) Angle (°) (Pa) Ethylene/octene 0.4522.9 29.2 2.6 × 10⁵ copolymer Ethylene/butene 0.27 21.7 27.3 3.3 × 10⁵copolymer

Example 15

[0089] In this Example, polyoctene was used as the release materialprecursor. This polymer was obtained starting from 1-octene by acoordinated anionic polymerization using a Ziegler.Natta catalyst. Morespecifically, a glass-made reactor having a stirring bar and a condenserwas dried. Into this reactor, argon was filled, cooling water wascirculated into the condenser, and reactants of 132 parts of 1-octene,394 parts of toluene and 0.54 ml of diethylaluminum chloride (1.8M intoluene) were fed. The reaction was initiated by adding 0.10 g ofAATiCl₃ (aluminum activated reducing titanium chloride, Stauffercatalyst). During the reaction, the temperature was kept at 25° C. Afterthe reaction for 5 hours, the polymer was precipitated with large excessmethanol and the remaining catalyst was extracted. Subsequently, themethanol solution was transferred to another vessel and the precipitatedpolymer was separated and dried in a vacuum oven.

[0090] The weight average molecular weight of the release materialprecursor of this Example was evaluated in accordance with Example 1 andfound to be 1,760,000. The obtained polyoctene was again diluted withtoluene to prepare a release material precursor solution having a solidcontent of 1 wt % and using this, a release sheet was manufactured andevaluated in the same manner as in Example 13.

[0091] The shear storage modulus of the release material precursor, therelease strength of the release sheet, the residual adhesion strength ofthe pressure-sensitive adhesive, and the contact angle of the releasesheet are shown in Table 17. It is seen from this Table that even when arelease sheet is manufactured from a polyolefin homopolymer, theobtained release sheet exhibits low release strength and is an excellentrelease sheet. TABLE 17 Residual Release Adhesion Contact StrengthStrength Angle Shear Storage Example (N/25 mm) (N/25 mm) (°) Modulus(Pa) Example 15 0.35 25.1 36.9 3.7 × 10⁴

[0092] According to the present invention, the release material has highanchoring to a substrate, the release strength from a pressure-sensitiveadhesive is relatively low even after the exposure to a hightemperature, and the pressure-sensitive adhesive can hold stableresidual adhesion strength.

What is claimed is:
 1. A release material formed by irradiating arelease material precursor, wherein said release material precursor hasa shear storage modulus of about 1×10² to about 3×10⁶ Pa, when measuredat 20° C. and at a frequency of 1 Hz, and wherein said release materialhas a contact angle of 15° or more, as measured using a mixed solutionof methanol and water (volume ratio: 90/10) having a wet tension of 25.4mN/m.
 2. The release material according to claim 1, wherein the releasestrength of the release material from an acrylic pressure-sensitiveadhesive, as measured at a release angle of 180° and at a release rateof about 300 mm/min, is about 0.05 to about 2 N/25 mm.
 3. The releasematerial according to claim 1, wherein said release material comprisesat least one member selected from polyolefins, poly(meth)acrylates andpolyvinyl ethers.
 4. The release material according to claim 3, whereinsaid polyolefin is formed from at least one olefin monomer having about2 to about 12 carbon atoms.
 5. The release material according to claim3, wherein said poly(meth)acrylate is a copolymer derived from a monomercomponent comprising at least one first alkyl (meth)acrylate containingan alkyl group having about 12 to about 30 carbon atoms and at least onesecond alkyl (meth)acrylate containing an alkyl group having 1 to about12 carbon atoms.
 6. The release material according to claim 3, whereinsaid poly(meth)acrylate is a homopolymer derived from a monomercomponent of an alkyl (meth)acrylate containing an alkyl group havingabout 8 to about 30 carbon atoms.
 7. A release material articlecomprising a substrate having release material layer on at least aportion thereof, wherein the release material comprises the releasematerial of claim 1, and wherein said release material layer is formedby coating said release material precursor on a substrate and thenirradiating a radiation said release material precursor.
 8. The releasematerial article according to claim 7, wherein said release materiallayer has a thickness of about 0.01 to about 1 μm.
 9. The releasematerial article according to claim 7, wherein a pressure-sensitivelayer is provided on said release material layer.
 10. The releasematerial article according to claim 7, wherein said substrate comprisesa polyester-based film selected from polyethylene terephthalate,polyethylene naphthalate, and polybutylene terephthalate.
 11. Therelease material article according to claim 9, wherein saidpressure-sensitive layer comprises an acrylic pressure-sensitiveadhesive, which acrylic pressure-sensitive adhesive comprises a(meth)acrylic resin.
 12. A process for producing a release materialarticle, the process comprising a step of: coating a release materialprecursor onto a substrate, wherein the release material precursor has ashear storage modulus of about 1×10² to about 3×10⁶ Pa, when measured at20° C. and at a frequency of 1 Hz, and a step of: irradiating saidrelease material precursor to form a release material layer having acontact angle of 15° or more, as measured using a mixed solution ofmethanol and water (volume ratio: 90/10) having a wet tension of 25.4mN/m.